Irregular heartbeats, particularly those during more intense physical activity, aren't often a cause for alarm. Most of the time, it's linked to heart palpitations, which are simply sensations that the heart is beating abnormally. Your heart is still working effectively. However, when this fluttering or pounding sensation is accompanied by other symptoms, it may be an indication of something serious. If you're concerned about your heart and its rhythm, don't hesitate to contact a doctor.
Heart palpitations during exercise are often related to the intensity of your workout. Catecholamines -- also known as "fight-or-flight" hormones -- tend to increase with vigorous exercise. The surge of hormones combined with the increased heart rate from physical activity can cause the heart to beat much faster than normal, resulting in sensations like skipped, fluttering or pounding heartbeats.
It's also possible that the palpitations are caused by an arrhythmia. In some people, the increased cardiac output from exercise can exacerbate a preexisting arrhythmia that doesn't normally present any symptoms. It isn't until the heart rate increases that the electrical impulses responsible for contracting your heart begin to misfire, so to speak, and you begin to feel the fluttering, pounding or skipped heartbeats.
Irregular heartbeats during exercise can sometimes be caused by a heart murmur. A heart murmur is basically an additional noise that occurs as the heart takes a beat. Instead of the two-beat sound, your heart makes another. Normally, you don't feel this additional rhythm, but exercise can exacerbate the murmur to the point where you become aware of it, much like an arrhythmia. You may then feel as if your heart is fluttering, pounding or skipping a beat.
If the irregular heartbeats are accompanied by chest pain, severe dizziness, severe shortness of breath or fainting, seek medical attention immediately. This could be an indication of a serious heart problem. A doctor can better determine the cause of the palpitations.